1. Field of the Invention
The present invention relates to a semiconductor substrate for a production of semiconductor transistor devices or photonic devices, and more particularly, to a compound semiconductor substrate comprising a single-crystalline semiconductor wafer substrate, a compound semiconductor epitaxial layer, and a strained layer superlattice (SLS) structure layer between the wafer layer and the epitaxial layer.
2. Description of the Related Art
Generally, a single-cyrstalline wafer of a binary compound semiconductor, such as GaAs and InP is used as a substrate for epitaxial growth of a compound semiconductor layer thereon.
For the epitaxial growth of a good quality compound semiconductor layer on the wafer substrate, it is necessary to coincide the lattice constant of the compound semiconductor epitaxial layer with that of the wafer substrate, which considerably limits the composition range of the epitaxial layer.
Recently, it has become necessary to form (grow) a compound semiconductor layer having a different lattice constant from that of the wafer substrate for semiconductor lasers having a relatively short emission wavelength. On the other hand, proposals have been made for producing GaAs grown on Si (GaAs/Si) substrates corresponding to a single-crystalline GaAs substrate, enlarging the wafer size, increasing the mechanical strength and thermal conductivity, and reducing costs, compared with the single-crystalline GaAs substrate. The GaAs has a lattice constance larger than that of Si, by about 4%.
Where such a compound semiconductor layer including the GaAs layer is grown, an SLS structure layer is integrated as a buffer between the substrate and the grown layer, for the following reasons:
(1) The SLS buffer layer reduces dislocations in the grown compound semiconductor layer by preventing an extension of threading dislocations from the substrate into the compound semiconductor layer, due to the strain field in the SLS; and
(2) The SLS buffer layer allows the epitaxial growth of the compound semiconductor layer having a different lattice constant from that of the substrate by absorbing a lattice mismatch without generating threading dislocations, due to an alternate expansion and contraction of strained very thin layers of SLS.
However, SLS buffer layers having both the satisfactory effects of (1) filtering of dislocations and (2) compensation of a lattice mismatch, have not yet been developed.
FIGS. 1A, 1B and 1C are fragmented sectional views of trial compound semiconductor substrate comprising at least a single-crystalline compound semiconductor wafer substrate, an SLS buffer layer, and an epitaxial compound semiconductor layer.
In these drawings, 1 indicates a GaAs (single-crystalline) wafer substrate having a lattice constant a.sub.s, 2 indicates an In.sub.x Ga.sub.1-x P graded layer, the component ratio "x" of which decreases from the wafer substrate side upward, 3 indicates an SLS structure layer of In.sub.0.24 Ga.sub.0.76 P-In.sub.0.76 Ga.sub.0.24 P, and 4 indicates an epitaxially grown In.sub.0.3 Ga.sub.0.7 P layer having a lattice constant a.sub.e. Reference 5 indicates an SLS structure layer having a lattice constant a.sub.1 approximate to that (a.sub.s) of the wafer substrate 1, 6 indicates an SLS structure layer having a lattice constant a.sub.2 approximate to that (a.sub.e) of the In.sub.0.3 Ga.sub.0.7 P layer 4, and 7 indicates generated threading dislocations. Reference 8 indicates an SLS structure layer having the same lattice constant as that of the GaAs wafer substrate, and 9 indicates an epitaxially grown In Ga P layer having the same lattice constant.
In the first compound semiconductor substrate shown in FIG. 1A, the lattice constance a.sub.s of the wafer substrate 1 is not equal to the lattice constant a.sub.e of the In.sub.0.3 Ga.sub.0.7 P layer 4 and the In.sub.x Ga.sub.1-x P graded layer 2 compensates this lattice mismatch. The wafer substrate 1 has a dislocation density on the order of .about.10.sup.4 cm.sup.-2, and the graded layer 2 has a dislocation density on the order of .about.10.sup.8 cm.sup.-2, since dislocation are newly generated in the graded layer 2. The SLS structure layer 3 reduces the dislocations, but the dislocation density is still on the order of .about.10.sup.6 cm.sup.-2. Since dislocation in the epitaxial grown In.sub.0.3 Ga.sub.0.7 P layer 4 follow the dislocations in the SLS structure layer 3, the layer 4 has a dislocation density on the order of .about.10.sup.6 cm.sup.-2. Furthermore, the compound semiconductor substrate is warped to some extent.
In the second compound semiconductor substrate shown in FIG. 1B, the lattice constants have the relationship "a.sub.e &gt;a.sub.2 &gt;a.sub.1 &gt;a.sub.s or a.sub.e &lt;a.sub.2 &lt;a.sub.1 &lt;a.sub.s ", and dislocations are unavoidably newly generated in the lower SLS structure layer 5. Although the upper SLS structure layer 6 reduces a dislocation density, the In.sub.0.3 GA.sub.0.7 P layer 4 has a dislocation density about 2 orders higher in magnitude than that of the wafer substrate 1. In this case, warping of the second compound semiconductor substrate is remarkably reduced.
In the third compound semiconductor substrate shown in FIG. 1C, since the SLS structure layer 8 substantially reduces dislocations, the epitaxially grown In Ga P layer 9 has a remarkably reduced dislocation density. Nevertheless, the lattice constants of the wafer substrate 1, and the layers 8 and 9 are the same, and thus the SLS structure layer 8 has the filtering effect but no compensation effect.
As mentioned above, interposition of an SLS structure layer or combined SLS structure layers between the wafer substrate and the epitaxial compound semiconductor layer cannot simultaneously attain the two above effects.